1. Field of the Invention
The present invention is broadly concerned with extrusion systems and methods characterized by a minimum of down time between individual runs and with comparatively little waste of starting materials during the course of the runs. More particularly, the invention is concerned with such methods and systems wherein the extrusion systems include a preconditioner and an extruder coupled in series with a variable speed discharge device therebetween; in use, the systems are run so as to maintain the discharge device in a full choke condition for as long as possible so that the extruder receives material at a continuous and non-varying rate throughout substantially all of a given run. This allows proper processing of almost all of the starting material while permitting rapid clearing of the system so that a new run can be almost immediately commenced. In preferred forms, a staged vertical cascade-type dryer forms a part of the system and permits drying/cooling of the individual products from the extruder in a continuous and product segregated fashion.
2. Description of the Prior Art
Extrusion systems have long been used for the production of a variety of food and other products. For example, many pet and human foods are produced using such equipment. Many extrusion systems include a preconditioner and an extruder in series relationship. Dry materials are fed from a bin system into the preconditioner outlet, where the materials are moisturized and partially cooked through application of steam and/or water and intense mixing. Such preconditioning materials are then fed into the inlet of an extruder equipped with one or more elongated, axially rotatable augers and an endmost apertured extrusion die. In the extruder, the materials are subjected to intense heat, pressure and shear and are forced through the extrusion die for complete cooking and shaping. Thereafter, the extruded products are typically dried and cooled in a multiple-pass dryer.
While extrusion systems of this type are common, significant operational problems remain. One such issue is the amount of waste involved in any given production run. Specifically, at the start up of a run waste is generated while the system comes into equilibrium and essentially continuous flow rates, pressures, temperatures, and residence times are established. Even more significant, however, is the waste problem encountered at the end of an extrusion run. Thus, when the last of a quantity of starting material is fed to the preconditioner, there inevitably follows a period where the flow of material to the extruder falls off until the preconditioner is emptied. Normally, the product produced during this last run stage is unacceptable and must be discarded. When it is considered that preconditioners hold from 900-1800 pounds of material, it will be appreciated that the last-stage waste is significant.
The above problem may not be deemed overly serious where large production runs are involved. Thus, if a 40-ton run is scheduled, the loss of 1,000 pounds of starting material may be sustainable. However, there is an increasing tendency to schedule short production runs of 5,000 pounds or less. In such cases the loss of 1,000 pounds at the end of the production run is economically unacceptable. This problem is so acute that some processors report that they obtain only a 60% yield on 4-ton batch runs.
Another adverse factor in extrusion processing stems from the down time associated with run changeovers. That is, where a processor wishes to change over a given system between two different products, down times of an hour or more are not uncommon. Again, where large-volume runs are scheduled, an operator can live with long down times. However, if a series of short (e.g., 5-ton or less) runs are scheduled on a production day, it will be seen that the changeover problem becomes significant.
The short run phenomenon also has a potentially adverse consequence for the post-extrusion drying operation. That is, the end-stage extrudate from a first run must not be allowed to commingle with the first-stage product from the next succeeding run. Therefore, unless special steps are taken, the extruder must be shut down between runs to allow sufficient time for passage and clearance of all the extruded product through the dryer.
There is accordingly a need in the art for improved extrusion systems and processes which overcome the problems outlined above and provide a quick changeover capability while also minimizing product loss.
The present invention overcomes the problems outlined above and provides improved extrusion systems, components thereof, and methods. Broadly speaking, the extrusion systems of the invention include an extruder having an elongated barrel with at least one axially rotatable, flighted auger therein, with the barrel presenting an inlet and an outlet and a die mounted at the barrel outlet. Such systems also include a preconditioner having shiftable mixing elements therein and an inlet for receiving material and an outlet coupled with the extruder barrel for feeding preconditioning material to the latter. A bin assembly and a variable speed inlet feeder screw are also normally coupled with the preconditioning inlet for feeding starting materials to the preconditioner.
An important part of the extrusion systems of the invention involves the use of a variable speed, variable output discharge feeder such as a screw feeder between the preconditioner outlet and the extruder inlet. In order to maximize usage of starting material, the system is run so that the screw feeder is maintained in a choke full condition for as long as possible. In this way, a steady and substantially constant flow of preconditioned material is delivered to the extruder for final processing. In order to accomplish this end, the control for the system may be set so as to alter preconditioner operation toward the end of a given extrusion run. To give one example, where a horizontally oriented preconditioner is employed the system may be set up so that, towards the end of the run, the preconditioner operation is altered to maintain the choke full condition at the discharge feeder. Such alteration may involve reversing the rotation of the preconditioner mixing paddles to force more material forwardly to sustain the choke full condition.
In another related aspect of the invention, the extrusion systems include one or more detectors coupled with a microprocessor controller such as a programmable logic controller (PLC). A detector assembly is operatively associated with at least the preconditioner (and usually the bin assembly as well) in order to determine the flow rate of material passing therethrough. Preferably, the mass flow rate is determined, but a volumetric flow rate could also be measured. The controller can then adjust system operation to maintain constant flow to the extruder for as long as possible. Preferably, a first detector (preferably in the form of a weighing device such as a load cell) is coupled with the bin assembly for determining when substantially the last of a predetermined quantity of starting material has been fed to the preconditioner. A second detector (also preferably a load cell) is coupled with the preconditioner and the two detectors are used to determine the flow rate through the preconditioner.
In order to further minimize down time, the systems of the invention preferably include a specialized multiple-position die assembly which can be rapidly shifted between first and second separate dies without the need for laborious changeovers which stop production. The preferred die assembly of the invention includes a head assembly including first and second spaced outlets with individual die members coupled thereto. A shiftable member such as a cylindrical rotor is located within the head and includes an elongated product-conveying passageway presenting a product inlet opening adjacent the extruder barrel outlet, and a spaced product outlet. A drive is connected to the shiftable member and is operable to selectively move the passageway outlet between the first and second dies. In addition, a third discharge outlet is preferably provided between the die outlets. The drive can move the passageway outlet to the discharge position during the hiatus between product runs so that the extrusion system can be flushed and unwanted extrudate discarded.
The extruder systems of the invention also preferably includes a dryer (preferably a multiple-stage cascade dryer) which receives extrudate from the extruder for drying and cooling. The operation of the dryer is correlated with the extruder operation so that products can be continuously dried but in a segregated fashion, i.e., the product from a first extrusion run is dried and maintained separate from the extrudate from a second extrusion run. Advantageously, the dryer is operatively coupled to the PLC for the extruder and related equipment.
Use of the systems and methods of the invention affords numerous advantages. Primary among these is the ability to process a salable product using a very high proportion of the starting materials. For example, where short product runs of up to about 10,000 pounds are performed, at least 90% and more preferably at least about 95%, and most preferably at least about 97%, of the starting material is converted into salable product. This proportion far exceeds the short-run yield obtainable with conventional systems. Second, the systems and methods of the invention allow a very rapid changeover between individual products. This is especially the case when use is made of the preferred control apparatus and die assembly hereof. Third, the systems of the invention permit the user to vary the residence time of material in the preconditioner during the course of an extrusion run while maintaining a constant output to the extruder.